1. Field of the Invention
This invention relates to methods and apparatus for forming seamless cylindrical photosensitive elements of uniform thickness on flexible sleeves and, in particular, to methods and apparatus for forming seamless cylindrical photosensitive elements of uniform thickness on flexible sleeves without sanding, grinding or additional polishing apparatus.
2. Description of Related Art
Flexographic printing plates are well known for use in printing, particularly on surfaces which are soft and easily deformable, such as packaging materials, e.g., cardboard, plastic films, etc. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,749. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable solvent removes the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing.
The flexographic photopolymerizable compositions can be formed into sheets or layers by several known methods such as solvent casting, hot pressing, calendering and extrusion. A preferred method of manufacturing flexographic printing elements is by extrusion calendering of the photopolymerizable composition. In extrusion calendering, the printing element is prepared by passing a mass of hot photopolymerizable material into an extrusion die forming a layer, passing the layer into the nip of a calender and, while still hot, calendering the photopolymerizable composition between two flat surfaces, generally two flexible films, to form a multilayer web. The films can include multiple layers or compound films. A film bearing a thin layer of flexible, polymeric material is an example of a compound film. After extrusion and calendering at elevated temperatures, the web is held in tension in the machine direction by a pair of nip rollers while the multilayer web is cooled, for example, with blown air. The printing element as a multilayer web can be cut into suitable size sheets. Extrusion and calendering of polymeric compositions are disclosed, for example, in Gruetzmacher et al., U.S. Pat. No. 4,427,759; and in Min, U.S. Pat. No. 4,622,088.
Although typically photopolymeric printing elements are used in sheet form, there are particular applications and advantages to using the printing element in a continuous cylindrical form. Continuous printing elements have applications in the flexographic printing of continuous designs such as in wallpaper, decoration and gift wrapping paper. Furthermore, such continuous printing elements can be well-suited for mounting on laser exposure equipment where it can replace the drum or be mounted on the drum for laser exposure as disclosed in U.S. Pat. No. 5,223,359 and U.S. patent application Ser. No. 08/432,411.
The formation of "seamless," continuous printing elements can be accomplished by several methods. The photopolymerizable flat sheet elements can be reprocessed by wrapping the element around a cylindrical form, usually a printing sleeve or the printing cylinder itself, and fusing or joining the edges together to form a seamless, continuous element. Processes for joining the edges of a plate into a cylindrical form have been disclosed, for example, in German patent DE 28 44 426, United Kingdom patent GB 1 579 817, and U.S. Pat. No. 4,758,500. A problem with the prior methods of joining the edges to form the continuous cylinder is that the results of printing with the joined cylinder are often unsatisfactory, particularly when the joined edge falls within the effective printing area of the plate. The joined seam is visible in and interrupts the printed image.
Further, photosensitive resin cylinders are also made from a layer of photopolymeric composition by a Seamex process. The Seamex process involves wrapping a layer of photopolymeric material to a nickel sleeve having a heat-activated primer coat to bond with the material so that the ends of the plate are joined together. The entire assembly is placed in an oven to cure and bond the photopolymeric layer to the primer coat and melt ends of the photopolymeric layer together. The photopolymeric layer on the sleeve is then ground to the necessary thickness, wiped clean and sprayed with a protective coating to prevent negatives from sticking to the photopolymer during exposure. The process of wrapping, curing and melting, grinding and spraying the photopolymeric layer to the sleeve takes about 1.5 to 2 days to accomplish. The cylindrical photopolymeric layer on the sleeve then undergoes the steps of imagewise exposure to polymerize those areas of the layer exposed to light, and processing to wash away the unpolymerized areas of the layer on the cylinder, forming a relief surface for printing.
Also, U.S. Pat. No. 4,337,220 discloses a process for preparing photosensitive resin cylinders which comprises winding a photosensitive resin sheet on and around the surface of a cylinder without any material overlapping of or spaces between the edge portions of the wound resin sheet, and applying to the cylinder under rotation a roll rotating in contact with the surface of the resin sheet while heating. The edge portions of the resin sheet join to one another by melting and the thickness of the resin sheet is made uniform. Preferably, the photosensitive resin sheet is wound on the cylinder with the aid of adhesive tape or agent. The resin sheet while on the cylinder is heated only in an amount sufficient for softening the resin sheet to avoid flowing of the resin because the flowing resin adheres to the roll or hangs down to make the control of the film thickness difficult.